Please anyone help me to understand how to choose good prop - motor and esc combination for quadcopter .  

hello @snchavan555 welcome to forum..
what type of help you want...??

regards

i wan to know about best combination of esce- motor - prop and how to calculate their thrust

what is your budget..

regards

right now max 20k

than in my opinion go for RTF kit from  www.quadkopters.com

that seems best for you. as you are beginner. first try your hand on kk FC .. best for beginner and for learning quad.

regards

www.quadkopters.com  has good ARF quad kits 

  www.quadkopters.com

another  option is www.rcmumbai.com  which is also a trusted reseller....

Are those rtf kits can be used to build good light weight video system with gimbal and all

For adding a gimbal and camera, you can go with 450 ARF kit, available at www.rcmumbai.com for only 6000/-

Thank you for recommending rcmumbai
Sooraj and madaquif

Is fs-t6 is good or i should buy FS-9X 2.4ghz transmitter as one time investment , for both basic quad and video sys  ?
and do you recommend any cheap but quite good transmitter ?

For video quad you surely need the 9ch.

Is fs-th9x is good or should i go for turnigy 9x. And which one is having good range ?

Flysky is the original manufacturer of TH-9X, and Turnigy is a re branded one.

Both the radio will give same range (approx. 1.5Km LOS)

you can go for which ever is cheaper (I think flysky is cheaper than Turnigy)

Flysky is better too.

Thanks.

Is there any problem with customs like importing transmitters in india if i only buy motors from hobbyking or some other website .?

No problems on getting motors...but if its a expensive motors be sure to ask the seller to mark the price about 50-60% of original price as they can attract customs